Alphabetically Union List - O
- O and C Navanakhon Labour Union
- OA Labour Union
- OC and Computer Labour Union
- Off Road Accessories limited Union
- Office of the Rubber replanting Aid Fund-state-enterprise worker union (ORRAFC-SEUUU)
- Office of the Rubber Replanting Aid Funf Labour Union
- Office Service Workers Union
- Offshore Petroleum Labour Union
- Oil and Gas Wellhead Platform Thailand Labor Union
- Oil Transport Workers Union
- Oishi Navanakhon Labour Union
- Oki Labour Union
- Olan Tour Thailand Labour Union
- Ong Sopisai Labour Union
- Orchid Bags Labour Union
- Orient Printing and Dying Labour Union
- Oriental Sports Labour Union
- Orora Mecatonics Labour Union
- Osaka Labour Union
- OSI Labour Union
- OTG Thai Labour Union
- Ovaltine Labour Union of Thailand
- Overseas Product Inspection Workers Union
